It looks as if your JSON menu file is not accessible at the URL you are using. You need to amend the URL or the file location (or both). For example:
/config/www/garmin/<file>.json maps to https://<domain>/local/garmin/<file>.json
This is covered in the basic setup in the README.md → Dashboard Definition.
Note I am not using Nabu Casa. There is a note on that page subsequently about changes to Nabu Casa that have been reported but which the authors are unable to verify:
Important
However recent reports suggest this path may no longer work on Nabu Casa and you should use
/homeassistant/www/instead of/config/www/. We are unable to verify this since our free trial of Nabu Casa has expired.
If this report is correct then try:
/homeassistant/www/garmin/<file>.json mapping to https://<domain>/local/garmin/<file>.json
Just a small refinement in the setup that’s occured. Once you get the file location correct, the online editor will be able to fetch your menu file via the arrow button. I expect this is why your watch gets no menu too.